Various aluminate phosphors have been put to practical use as phosphors for energy-saving fluorescent lamps. Examples of blue phosphors include (Ba,Sr)MgAl10O17:Eu (BAM:Eu), and examples of green phosphors include CeMgAl11O19:Tb and BaMgAl10O17:Eu,Mn.
In recent years, BAM:Eu, which has high luminance under vacuum-ultraviolet excitation, has been used as a blue phosphor for a plasma display panel (PDP).
However, when a light-emitting device, especially one using the blue phosphor BAM:Eu, is driven for a long period of time, the luminance is degraded significantly. Hence, for use in a light-emitting device, particularly in a PDP, there is a strong demand for a phosphor that shows less luminance degradation even after the long-term driving.
In response to this demand, methods using certain kinds of silicate phosphors for light-emitting devices are proposed. For example, JP 2003-132803 A and JP 2004-176010 A disclose a method using (Sr1-a,Baa)3-dMgSi2O8:Eud (where 0≦a≦1 and 0.01≦d≦0.1). JP 2006-12770 A discloses a method using M3-eMgSi2O8:Eue (where M is at least one element selected from a group consisting of Sr, Ca and Ba, and 0.001≦e≦0.2).
However, the intensive studies of the present inventors have revealed that the light-emitting devices using the phosphors described in the above-mentioned documents, in most cases, cannot achieve inhibition of the luminance degradation of the phosphors during driving while maintaining high luminance. Moreover, these methods have the following problems. When a Sr site is not replaced with Ba, the chromaticity y is higher and the color purity is worse than those of a blue phosphor BAM:Eu that is currently used in the PDP. On the other hand, when the Sr site is replaced with Ba, the emission luminance drops significantly.